1. Field of the Invention
The present invention relates to solid phase welding. Primarily, the invention is concerned firstly with a method of repairing an aircraft skin such as an aerofoil by solid phase welding and, secondly, with a solid phase welding apparatus
2. Discussion of Prior Art
It is of vital importance that any damage incurred by an aerofoil of an in-service aircraft is repaired immediately and effectively. It is not unusual for cracks to occur in the skin of a leading edge of an aircraft wing. The leading edge of an aircraft wing may be made from a flat sheet of suitable light alloy such as 2024 aluminium alloy of about 3 mm thickness.
The current method of repair of such leading edge cracks involves preventing the crack from extending any further by drilling a hole at each of its ends and then reinforcing the area in the immediate vicinity of the crack, by fastening a plate or bracket over the crack to reduce the level of stress incurred in that area. The disadvantage with such a method is that the repaired area must be continually monitored and frequently inspected to make sure the repaired area is always capable of withstanding the stresses and strains on the aircraft wing when the aircraft is in service.
Traditionally, welding has not been considered as a suitable method of repair of an aircraft skin. This is primarily due to the high residual stress produced at weld toes, caused by the contraction of the material during the welding process. This, in turn, results in increased fatigue in the repaired area, as well as increased crack propagation life. Furthermore, it is extremely difficult to avoid distortion of the thin light alloy material during the welding process.
One object of the present invention is to provide an improved method of repairing an, aircraft skin, such as used on an aerofoil.
According to a first aspect of the invention there is provided a method of repairing an aircraft skin comprising introducing a solid phase welding tool between adjacent surfaces of the aircraft skin and operating the tool so as to weld the adjacent surfaces together.
The applicants have discovered that solid phase welding may provide a suitably strong weld, and can be used as a method of repair of a crack in an aircraft skin while not jeopardising the properties required throughout the material to provide optimum safety, particularly when the repaired region is subjected to the stresses normally associated with an aircraft during flight, take-off and landing.
The method preferably comprises using a welding tool in the form of a friction stir welding tool.
Friction stir welding is a form of solid phase welding used to produce lap and butt welds in aluminium sheets or plates. The process normally involves plunging a rotating tool slowly into a joint line between two sections of material that are clamped in position to prevent movement. Frictional heat is generated by the welding tool which plasticises the material directly under the tool head. The desired length of weld is achieved by traversing the tool along the weld line and stiring the softened material together. The material does not reach its melting point at any time during the welding process.
The friction stir welding process has many advantages over traditional welding techniques. Most notably, the welded material undergoes very little distortion or shrinkage during the process. The process does not produce any fumes, sparks, porosity or spatter. The process can be repeatedly carried out, efficiently and effectively, on materials that are known as being difficult to weld (such as, for example, 2000 and 7000 series aluminium alloys). Furthermore, the process is energy efficient and suitable for automation. The welding tool is generally non-consumable, and can often be used for up to 1000 m of weld length during its lifetime, before requiring replacement. No filler or gas shielding is required. The mechanical properties of the welds produced are also substantially improved. Friction stir welding provides ease of repair and good post-repair benefit, while avoiding the disadvantages of increased fatigue and stress to the repaired area, due to the fact that the welding is carried out in the solid phase and at a low temperature.
Although friction stir welding is known, per se, as a technique, for joining two metal plates, or two sides of a metal plate, it has never been used before as a method of repairing crack damage in an aircraft skin. Moreover, it has not been considered as a possible method of repair of crucially important components such as an aircraft wing. This is because general welding techniques, particularly on thin light alloy skins used on aircraft have been problematic due to the increased fatigue of the repaired section once it has been subjected to heat, and the material itself has undergone melting. Furthermore, an aircraft would usually be required to be taken out of service in order for any such welding to take place. The aircraft would need to be taken to a specific area where special safety conditions have to be met, in view of the intense heat and sparks that are produced, in order to ensure that the actual welding process could be carried out as safely as possible.
In the preferred embodiment, the method comprises introducing the solid phase welding tool into a crack in the aircraft skin, the adjacent surfaces being defined by edges of the crack. The solid phase welding tool may be introduced at one end of the crack and then moved along the crack to create the weld.
The method may comprise moving the solid phase welding tool through a hole formed in the aircraft skin during the welding operation. The method may also comprise re-forming the hole after the welding has been completed.
The method may comprise positioning a metal element at an edge of the skin to be repaired and moving the solid phase welding tool beyond the edge and into the metal element at the end of the welding process. The metal element can then be removed and the weld at the edge of the skin can be fettled to provide a clean finish.
Preferably, the method comprises positioning the solid phase welding tool so as to extend part way through the thickness of the aircraft skin. In that way, the position of the tool effectively controls the depth of the weld.
Previously, solid phase welding apparatus, and in particular friction stir welding, apparatus has been of a non-portable kind because of problems of workpiece clamping and access requirements. The present invention has addressed this problem and, in that respect the method may comprise mounting a solid phase welding apparatus to the aircraft skin, carrying out the solid phase welding and then removing the solid phase welding apparatus from the aircraft skin.
According to a second aspect of the invention there is provided solid phase welding apparatus comprising a base, a support for a solid phase welding tool movable on the base, and means for mounting the apparatus on a workpiece to be welded, the movement of the support on the base enabling the tool to be moved relative to the workpiece as the tool is operated to produce a weld.
As the solid phase welding is a low temperature process, safety precautions regarding the production of heat and sparks are not as stringent as for general welding techniques. Therefore, it is possible to transport the apparatus, say, to an aircraft, rather than the other way around, and to carry out the friction stir weld on the aircraft while the aircraft remains in its usual resting place.
Preferably, adjustment means is provided on the support for moving the tool towards the workpiece and enabling a desired depth of weld to be produced in the workpiece. Locking means may be provided which locks the tool in a predetermined position to provide the desired depth of weld. The locking means may comprises a locking pin which may be biased towards a locking position. The locking means may be releasable from its locking position by moving it against the bias.
The support may comprise a first part which is movable on the base and a second part such as an arm which is movably mounted on the first part and which is arranged to carry the welding tool. Preferably, the arm is pivotally connected at one end to the first part and is arranged to carry the tool at its other end. Where the aforesaid adjustment means is provided, it may be arranged to control the position of the second part and thereby the position of the tool.
The said adjustment means may be operable by a depth control actuator.
The support may be movable on the base by means of a support actuator.
The depth control actuator and the support actuator are preferably in a control circuit whereby when one of the actuators reaches a given position, the other actuator begins to operate. For example, the support actuator may begin to move the support after the depth control actuator has moved the tool to the desired depth.
Preferably, clamp means is provided for securing the apparatus to the workpiece. In such a case, the clamp means may be mounted to enable the workpiece to be clamped against the base.